The DSL technology is a high-speed transmission technology, where data is transmitted through a telephone twist pair cable, that is, an Unshielded Twist Pair (UTP) cable, and the DSL includes an Asymmetrical Digital Subscriber Line (ADSL), a Very-high-bit-rate Digital Subscriber Line (VDSL), an Integrated Services Digital Network (ISDN) Digital Subscriber Line (IDSL), and a Single-pair High-bit-rate Digital Subscriber Line (SHDSL), which are collectively called xDSL in the following.
In the X Digital Subscriber Line (xDSL) technologies, besides the DSLs being the IDSL and SHDSL, where transmission is performed through a baseband, the DSL using a passband for transmission uses a frequency division multiplexing technology, so that the DSL and a Plain Old Telephone Service (POTS) coexist on the same pair of the twist pair cables, where the DSL occupies a high band, the POTS occupies a baseband part lower than 4 KHz, and a POTS signal and a DSL signal is split or integrated through a splitter. The xDSL using the passband for transmission uses a Discrete Multi-Tone Modulation (DMT) technology to perform modulation and demodulation. A system providing multi-path DSL access is called a DSL Access Multiplexer (DSLAM), and a schematic view of system connection relations of the DSLAM is as shown in FIG. 1. A subscriber end DSLAM 120 includes a subscriber end transceiver unit 121 and a subscriber end splitter 122. In an upstream direction, the subscriber end transceiver unit 121 receives a DSL signal from a computer 110, amplifies the received DSL signal, and sends the amplified DSL signal to the subscriber end splitter 122. The subscriber end splitter 122 integrates the DSL signal from the subscriber end transceiver unit 121 and a POTS signal from a telephone terminal 130. The integrated signal is transmitted through a multi-path UTP 140, and is received by a central office end splitter 151 of a peer end DSLAM 150. The central office end splitter 151 splits the received signal, sends the POTS signal in the received signal to a Public Switched Telephone Network (PSTN) 160, and sends the DSL signal in the received signal to a central office end transceiver unit 152 of the DSLAM 150. The central office end transceiver unit 152 amplifies the received signal, and sends the amplified signal to a Network Management System (NMS) 170. In a signal downstream direction, a signal is transmitted in an order reverse to the above order.
A subscriber cable generally includes multiple (25 or more) pairs of twist pair cables. Various services may be run in each twist pair cable. When different kinds of xDSLs are run at the same time, crosstalk is incurred among the different kinds of xDSLs, and performance of some subscriber lines may drop sharply due to the crosstalk. If the subscriber lines are long, no DSL service of any kind can be run in some subscriber lines due to the crosstalk. The crosstalk is a major factor affecting a subscriber rate in a VDSL system. Especially for long and short lines, as shown in FIG. 2, upstream far-end crosstalk (FEXT) from a short line to a long line is very serious, after a short line is activated, performance of an already activated long line drops sharply, and the long line may even be dropped.
UPBO is configured to reduce the upstream FEXT from a short line to a long line in the same bundle of lines. That is to say, through appropriate configuration, upstream Power Spectral Density (PSD) of short lines is appropriately decreased, so as to reduce the crosstalk to long lines in the same bundle, so as to enhance stability of the operation of the subscriber lines.
In Chapter 7.2.1.3 of International Telecommunication Union-Telecommunication (ITU-T) Recommendation G993.2, an existing UPBO method is proposed.
In the existing UPBO method, a set of formulas and a reference point determined by an operator are defined, each subscriber line automatically calculates an UPBO PSD mask of each subscriber line itself. That is to say, each subscriber line calculates the upstream PSD of each subscriber line itself according to the subscriber line length, which is represented by the length of a circuit. In particular, a formula used by each subscriber line to calculate the UPBMASK is as follows.K(kl0,f)=UPBOPSD(f)+LOSS(kl0,f)+3.5 [dBm/Hz],  (1)
where,
LOSS(kl0,f)=kl0√{square root over (f)} [dB]
UPBOPSD(f)=−a−b√{square root over (f)} [dBm/Hz],
UPBOMASK (kl0, f) represents the UPBOMASK of the line segment on a band f; LOSS (kl0, f) is an attenuation value; kl0 is circuit length of the line segment; a and b are UPBO parameters, a is used for representing original point information of the back-off of the band, which is uniformly called parameter a or an original point parameter hereafter; b is used for representing back-off slope information of the subscriber line on the band, which is uniformly called parameter b or a back-off slop parameter hereafter.
In the method, reference line length kl0-ref is selected firstly according to subscriber line scenario experience, a parameter a(i) and a parameter b(i) of each pair of lines on each band are then calculated according to the reference line length, a Customer Premise Equipment (CPE) modem calculates the UPBOMASK of the back-off of whole upstream band according to formula (1), and the UPBOMASK is intersected with a pre-defined standard PSDMASK. That is to say, power back-off is performed on all bands of subscriber lines in a bundle of lines shorter than the reference line length according to a and b configured by a template.
When the values of the parameter a(i) and the parameter b(i) of each pair of lines on each band are calculated according to the selected reference line length, the calculation is performed by reasonable deduction with the standard PSDMASK value, the reference line length, and the above formula. More details may be obtained from ITU-T Recommendation G.993.2 and other documents, and are not described herein.
During the implementation of the present invention, the inventors find that the existing UPBO method has the following defects.
If the selected reference line is too short, many lines are not covered by the performing of the UPBO, the FEXT from a close line shorter than the reference line to a farther line is still very strong, so that the performing of the UPBO does not achieve an obvious effect. If the selected reference line is too long, the UPBO is performed on all of the lines, the FEXT among lines is not strong, but since power of a high frequency part of a short line is lowered enormously, a great loss is incurred to an upstream rate of the short line.
In Amendment 2 of ITU-T Recommendation G.993.2, an equalized UPBO method, that is equalized FEXT UPBO, is proposed. Since in the described existing UPBO method, the impact of the FEXT from short lines is over estimated, which results in too much back-off of spectrum of the short lines, a parameter of reference line electrical length kl0-ref is introduced in this method, and the UPBO is performed by adopting the following formula.
                                          UPBOMASK            ⁡                          (              f              )                                =                                    -              a                        -                          b              ⁢                              f                                      +                          10              ⁢                                                          ⁢                                                log                  10                                ⁡                                  (                                                            kl                                              0                        ⁢                        _REF                                                                                    kl                      0                                                        )                                                      +                          LOSS              ⁡                              (                                                      kl                    0                                    ,                  f                                )                                      +                          3.5              ⁢                                                          [                              dBm                /                Hz                            ]                                      ,                            (        2        )            
In the formula, 0<kl0<kl0—REF. In the method, compensation is performed on the short lines to some extent.
During the implementation of the present invention, the inventors find that the method also has the following defects. In the technical solution, an FEXT compensation item is added to prevent the PSD of short lines from being lowered too much. But on all bands, a value used for compensation is constant, the back-off is still performed on the short lines on the high band which is not used by the long lines, so that losses incurred to the short lines on the high frequency are still heavy.
In view of the above, in the two existing UPBO methods, since back-off is performed on the whole band according to the parameter a and the parameter b, the upstream rate of the short lines is decreased sharply, and performance of the short lines is harmed seriously. Specifically, since the back-off increases along with the rising of the frequency, the back-off is also performed on the short lines on the high band not used by the long lines. Although the short line may incur crosstalk to the long line on the high frequency part, the long line does not bear bit on the high band, that is to say, the back-off performed on the short line on the high frequency part does not improve the performance of the long line, but the performance of the short line makes sacrifices instead. Especially in current years, due to demands of high-speed services, such as Voice over Internet Protocol (VOIP), Internet Protocol television (IPTV), and High-definition television (HDTV), the trend of the convergence of three networks, becomes increasingly obvious, and the requirements on the bandwidth are increasingly higher.